PhD Thesis
Rift Valley fever (RVF) is an enzootic/endemic zoonotic disease affecting human and animals in Africa and caused epidemic in Saudi Arabia and Yemen. The RVF is caused by the Rift valley fever virus (RVFV) of the order Bunyavirales, family Phenuiviridae, genus Phlebovirus. Outbreaks of RVF have a devastating impact on human and animal health in Africa, with heavy economic loss due to the banning of livestock export and the loss of food meat, coupled with human infection. They are often characterised by large, sweeping abortion storms and cause significant mortality in young livestock. The aim of the current study was to determine the seroprevalence rate and inter-epizootic/epidemic RVFV seroconversions as evidence of infection by this virus in domestic ruminants and rodents in areas with no previous history of RVF outbreaks. Efforts to understand the mechanism of inter-epizootic/epidemic maintenance and transmission cycle of RVFV generated serological evidence in several African countries that support a role for domestic ruminants and rodents. However, in Tanzania, as well as other RVFV enzootic countries, evidence can be confounded by the detection of RVFV antibody in domestic ruminants that most likely reflected infection acquired during the 2006/07 epizootic/epidemic in Tanzania, Somalia and Kenya, or the serological results were not confirmed by the specific neutralizing test. The current study overcame this potential confounder by selecting cattle that were born after the 2006/07 outbreak and by employing the more specific neutralizing test to confirm the identification of the antibody as being induced by RVFV. The study was conducted in Kyela and Morogoro districts, Tanzania, from June 2014 to May 2018. Serum samples to determine RVFV seroprevalence rate in cattle (Bos indicus and their crossbreed with Bos Taurus), sheep (Ovis aries), goats (Capra hircus), rodents (Muridae) and shrews (Soricidae) were collected in a cross-sectional study, while sera samples for determining inter-epizootic/epidemic RVFV seroconversions in cattle, sheep and goats were collected in a longitudinal study. A total of 356 serum samples from cattle, 304 from sheep and goats and 581 serum samples from rodents and shrews were collected and analysed for RVFV antibody by competitive and IgM capture Enzyme-linked immunosorbent Assay and plaque reduction neutralisation test.
Overall RVFV antibody seroprevalence rate in cattle by cELISA in both districts was 29.2% (104 of 356) with seroprevalence rates of 33% (47/147) in the Kyela district and 27% (57/209) in the Morogoro district. In total, 8.4% (30/356) of all cattle sampled had RVFV IgM antibody, indicating current disease transmission. When segregated by districts, the RVFV IgM antibody seroprevalence rate was 2.0% (3/147) and 12.9% (27/209) in Kyela and Morogoro district respectively. Mikese ward hard the highest RVFV IgM antibody seroprevalence rate 36.8% (14/38), then Magadu 11.7 (12/103), Bujonde 2.9% (2/70), Katumba Songwe 2.5% (1/40). None of the cattle sera sample from Kajunjumele ward was positive for RVFV IgM antibody. When the 104 cELISA positive sera samples were analysed by PRNT80 to confirm that RVFV-specific antibody were present, the majority (89%, 93/104) had RVFV neutralising antibody. On the other hand, the RVFV seroprevalence rate in sheep and goats was 10.5% (32/304) with 4.9% (15/304) of all sera samples from sheep and goats being positive for RVFV IgM antibody. The RVFV neutralising antibody in small mammals was 8.8% (51/581) M. natalensis had the highest seroprevalence 8.9% (49/552), Crocidura spp 8% (2/25) and none of the sera from R. rattus was positive for RVFV neutralising antibody. The overall RVFV seroconversion rate was 25.7% (45/175). Eight cows that aborted during this period were positive for RVFV IgM antibody. This study demonstrated widespread RVFV inter-
outbreak circulation among domestic ruminants in areas without previous reports of outbreaks. The study also demonstrated RVFV neutralizing antibody in rodents and shrews, thus indicating the potential role of these mammal species in the inter-outbreak maintenance of the virus. In conclusion, the RVFV seroconversions among ruminants and small mammals provided evidence of active transmission of RVFV in the Kyela and Morogoro, district of Tanzania, and therefore, may represent an enzootic/endemic maintenance and transmission of RVFV during the inter-outbreak periods for this virus.